When I moved from 1.8.7 to 2.1.1 I ran into a weird issue where sub requests were not being processed and blocks were not being performed if you were using mod_limitipconn. You can read about it here. During those days, I was running into intermittent segmentation faults with one vhost. I discovered that if I commented out the ErrorDocument for that particular vhost, the segfaults went away. I didn't give much more thought on it, but when we just rolled out to 2.1.2, we received more of these... enough for me to start digging.
What I discovered, if a location was using a proxy (squid / dansguardian, etc) and posted a form that was using the multipart/form-data enctype, without supplying a upload file, the segfaults would occur. I immediately contacted mod_sec mailing list (I'm tellin' ya, they have the best mailing list and responses to bugs!!!). I started working with one of the developers where we were trying to understand the cause.